The preparation of multilayer extruded articles in the form of slabs, films, structural shapes, etc., is increasingly developing in the modern industry for the processing of plastic materials because of the technical and economical advantages involved.
The technique for obtaining the slabs, etc., allows to perform, by the suitable combination of different extruded polymers, multilayer composites having special practical performances which cannot be obtained by the use of a single polymer.
The above-mentioned multilayer composites may be obtained according to two different processes, that is by lamination or by coextrusion.
The lamination method which has been used till now for flat slabs, foresees the application on the slab, at the outlet of the extrusion head, of one or more performed films of other polymers by a slight deflection between the hot rollers of a rotary press.
The coextrusion method, on the contrary, foresees the simultaneous extrusion of many polymers which flow together to a single head wherein the stratified flows of the different components are coupled, thus giving the multilayer extruded article.
The production of coextruded composites is used for different purposes. For instance, to give specified surface effects (brilliance of opacity) to aesthetically poor materials. Or it is used to obtain composites showing particular properties at acceptable costs. Generally, there have been attempts to couple a carrier base polymer with one or more thin layers of valuable polymers able to give particular properties, such as, for instance, the resistance to external agents, barrier properties against moisture or gas diffusion, or a better resistance to abrasion or scratching of the article, etc.
The necessary condition to obtain a good multilayer composite is that the different polymers are compatible among them so that a good adhesion among the layers of the different polymers used is obtained.
In the event that the polymers used are not sufficiently compatible, the additional use of adhesive agents is employed and there is the problem of recovering scraps which weighs heavily negatively on the cost of the process.
Vinyl-aromatic polymers, such as anti-shock polystyrene, owing to a good processing balance, properties and cheapness, are attractive materials as carrier base resins for the production of multi-layer coextruded composites. However, their use as such is limited because of their incompatibility with some polymers such as, for instance, technopolymers and polar polymers in general. For instance, this is the case with the multilayer coextruded composites of anti-shock polystyrene (HIPS) with special technical polymers such as polymethylmethacrylate (PMMA), polycarbonate (PC), thermoplastic polyurethane (TPU), polybutyleneterephthalate (PBT), acrylonitrile-butadiene-styrene terpolymer (ABS) and the like.
Therefore, in the above mentioned case there is noted a poor adhesion of the layers with signs of delamination either on the coextruded slabs or on the manufactured articles obtained therefrom by thermoforming.